Application range straight seam steel pipe diameter range is 406~1600mm, even larger under special circumstances, the ratio of pipe wall thickness to pipe diameter is about 0.06~0.08. Foreign modern pipe making technology can produce welded pipes with wall thickness up to 100mm.

There are only a few methods for producing straight seam steel pipes from steel or steel strips in the industry. The principle difference lies in the forming method of the steel pipes. The subsequent processing of the steel pipes after forming is basically the same.

The forming method depends in principle on the distribution of the welds of the steel pipes. The steel pipes are divided into two basic forms according to the distribution of the welds: lsaw steel pipes and lsaw steel pipes. Correspondingly, the production of welded pipes is divided into the production of straight seam steel pipes and the production of spiral seam pipes. The most widely used welding method for straight seam steel tubes and spiral seam tubes is submerged arc welding (SAW), which has good quality, high productivity, mature technology and stability.

At present, foreign straight seam steel pipes are widely used in the following ranges:
· Main pipelines for the transport of gas, water and oil on land and in the sea;
· The chemical industry transports stainless steel pipes for chemical raw materials and products;
· Marine industrial structure pipe;
· Boiler manufacturing tubes and special purpose structural tubes.

Production of straight seam steel pipe
The production of straight seam steel pipes can be divided into two stages, namely the forming stage and the post-forming manufacturing stage.

The production of large-diameter straight seam submerged arc welded steel pipes in foreign countries is divided into four types according to the forming method:
· UOE process;
· Rollbending process;
· Progressive forming process;
· Progressive folding process.

Production of straight seam steel pipe
1.1 Forming stage
1.1.1 UOE forming
This method has the highest productivity and is the most important production method for straight seam steel pipes. The brief production process of the unit is first bent into a U shape, then pressed into an O shape, mechanical cooling expansion (Expansion) after internal and external welding.This method is characterized by the availability of extremely stable steel tubes with high productivity and the production of steel tubes over 18 m in length.

1.1.2 Roll forming
In this method, after several steps on the roll bending machine, the steel sheet is bent into an open tube, and the edge of the opening is not deformed and is still straight. Then open the tube is sent to the edge bender and the two edges of the plate are continuously rolled.

This method is characterized by strong adaptability and good economics for medium production, but the minimum diameter and maximum wall thickness of the steel pipe produced restricted.

1.1.3 Gradual molding
The forming process of this method is that the steel sheet is first sent to the pressing type, and then sent to the bending position by the controller, and an open tube is formed after passing through a series of pressing molds matched with the tube diameter. There are two controllers in operation, one side of the board is first bent into a semicircle, then the board is second controller moving, the other side is then also bent. Since the thickness of the curved die affects the roundness of the open tube, the thickness of the curved die must be kept to a minimum. Typically, after the forming is completed, the open tube is sent to the edge bender and the two edges of the sheet are continuously rolled into the desired shape. This method is characterized by strong adaptability, economical for medium production, and the ability to make small diameter and thick wall steel pipes.

1.1.4 Gradual bending forming
The steel tube forming by this method was originally used in the marine industry, and it is often necessary to form a particularly thick steel pipe by using a controlled rolling sheet in a cold state, and this method has high forming precision and pressure, and is used for making a sea. Industrial thick-walled steel pipe. This method is characterized by high-strength and thick-walled steel pipes, which are suitable for both small diameters and large diameters, so it can be manufactured in trunk pipes and marine structural pipes; system equipment is low in cost, adaptable, and economical. Good sex, even in small batch production, can reduce costs.

1.2 Manufacturing stage after forming
After the above forming stage is completed, the post-manufacturing stage of the large-diameter straight seam steel pipe includes a series of substantially similar processes, the main process:
1.2.1 The edge of the board is machined to weld the groove.
The machining methods are milling and planing. On either side of the board, there may be one or more milling and planing heads. According to the thickness of the plate, the groove can be processed into an I-shaped, single V or double V groove with a certain blunt edge. The extra-thick steel pipe can be used to mill the outer seam into a U-shaped groove. The purpose is to reduce the consumption of welding materials and increase the productivity, while the root is wider to avoid welding defects.

1.2.2 tack welding, commonly known as pre-welding. It is generally carried out by carbon dioxide gas shielded welding, the purpose of which is to stabilize the steel pipe, which is particularly useful for subsequent submerged arc welding to prevent burn through. After the steel pipe is positioned and welded, it shall be visually inspected to confirm that the weld is continuous without any defects.

2.2.3 Inner and outer welding of steel pipes, that is, precision welding. After the steel pipe is positioned and welded, the subsequent welding is mainly the inner and outer welding of the steel pipe, which is an important part of the steel pipe manufacturing process. It is completed by a submerged arc welding method separate from the forming unit. In order to improve productivity, the inner and outer seams are welded by multi-wire submerged arc welding, and the number of welding wires can be up to 5 wires. In order to avoid weld deviation, a special weld automatic centering device is installed on the welding head. Multi-layer welding is applied to thick-walled steel pipes to reduce heat input and improve the physical properties of the weld.

1.2.4 Weld inspection.
In order to identify the welding defects as soon as possible, ultrasonic testing and X-ray inspection are performed immediately after the welding operation is completed, and the defects are found to be repaired in time.

1.2.5 Cold expansion.
After the completion of welding, the roundness and straightness of the steel pipe usually cannot meet the relevant standards and technical requirements. The sizing and straightness are used in the pipe manufacturing plant and are completed by the mechanical cold expansion method.

1.2.6 Water pressure test of steel pipes.
The test pressure can be as high as 90% or more of the yield strength of the steel pipe material.

1.2.7 Finally, the entire steel pipe is subjected to ultrasonic inspection and X-ray inspection and visual inspection.